RFID tag and reader systems may operate over a wide range of frequencies, including low-frequency (LF) applications, high-frequency (HF) applications, and ultra-high-frequency applications (UHF). LF applications typically operate from 125-148.5 kHz. HF applications typically operate at 13.56 MHz. UHF applications typically operate from 300 MHz to 3 GHz. The “read range” of an RFID tag and reader system is often defined as the distance from which a reader can communicate with a tag. Passive LF and HF applications offer very short read ranges, often requiring the RFID tag to be within 0.01 to 0.5 m of a reader for successful communication. Passive UHF applications typically offer longer read ranges, allowing RFID tags to be within 2 to 12 meters or more of a reader for successful communication. In this case, the maximum read range is mainly limited by the sensitivity of the charge pump defined as the minimum input RF power to the charge pump that is required to deliver the required DC power needed by tag digital and analog circuits. The read range can be improved by two means: (a) reducing the DC power dissipation in the tag circuits, and (b) boosting the efficiency of the charge pump, while still meeting the bandwidth requirement.
FIG. 16 schematically shows a known one stage charge pump 1 using the so-called threshold VT cancellation. The bias voltages 2 required by the main rectifiers M1 and M2 are generated by “auxiliary” charge pumps 3, which are supplied by the same RF AC input 4. Unlike the main rectifier, the auxiliary pumps M1, M2 have only capacitive load (the gates of M1 and M2); therefore their biasing is much less critical. The rectifier M1 is “on” when the ac-coupled input signal is negative, while rectifier M2 is on when the input signal is positive. Thus a DC output 5 is provided. The quality factor of the charge pump has to be constrained to about 10 at maximum in order to meet the bandwidth requirement of the application. The restricted quality factor results in a significant power loss in the rectifiers. Therefore, the charge pump shown in FIG. 15 has a moderate efficiency of about 35%.
In view of the above-described situation, there exists a need for an improved technique that enables a charge pump to have a high efficiency while substantially avoiding or at least reducing one or more of the above-identified problems.